Voltage/current measuring unit and method

ABSTRACT

A voltage/current measuring unit includes a signal generating source having a digital error detecting unit and a digital-to-analog converter (DAC) connected in series, a current measuring resistor having first and second terminals respectively connected to an output terminal of the signal generating source and a connection terminal, a voltage measuring circuit connected to the digital error detecting unit for measuring a voltage at said second terminal of the current measuring resistor and for providing a voltage feedback signal to the signal generating source, a current measuring circuit connected to the digital error detecting unit for measuring a voltage across the first and second terminals of the current measuring resistor, and for providing a current feedback signal to said signal generating source, the voltage measuring circuit and current measuring circuit both include analog-to-digital converters for respectively converting a voltage appearing at the second terminal of the current measuring resistor and a voltage appearing across the first and second terminals of the current measuring resistor into digital voltage values that are fed to the signal generating source.

This is a continuation of application Ser. No. 08/195,383 filed on Feb.14 1994 now abandoned.

FIELD OF THE INVENTION

The present invention relates to a voltage/current measuring apparatusand method employed in electronic measuring apparatuses such assemiconductor measuring apparatus, and more specifically, to such avoltage/current measuring apparatus/method that is capable of measuringvarious electric characteristics of a device under test (DUT) .

BACKGROUND OF THE INVENTION

FIG. 6 is a schematic block diagram showing a prior art voltage/currentcharacteristic measuring unit (PMU) employed in conventionalsemiconductor testing apparatus (for instance, a semiconductorcharacteristic measuring apparatus such as an HP4145 manufactured by theHewlett Packard Company). The unit is capable of performing both voltagesetting/current measuring operations and current setting/voltagemeasuring operations, the architecture of which is widely used in ICtester and DC characteristic evaluation apparatus marketed by theApplicant's Assignee.

In FIG. 6, an error amplifier 111 is connected via an integrator 112 anda buffer 113 to one terminal "a" of a current measuring resistor 120.Signal generating source 110 comprises error amplifier 111, integrator112, and buffer 113.

The other terminal "b" of current measuring resistor 120 is connected toa predetermined terminal of a DUT. Both ends of resistor 120 areconnected via buffers 131a and 131b, respectively, to input terminals ofa differential amplifier 132. An output terminal of differentialamplifier 132 and an output terminal of buffer 131b are connected toerror amplifier 111.

It should be noted that the buffers 131a, 131b and differentialamplifier 132 constitute a current measuring circuit, whereas the buffer131b constitutes a voltage measuring circuit. When voltagesetting/current measuring operations are performed, a setting voltage(V_(FIN)) is applied as an analog voltage to error amplifier 111 from ameasuring signal process circuit (not shown) via a digital to analogconverter (DAC) (not shown). Error amplifier 111 receives a voltageV_(MOUT) appearing at terminal "b" of current measuring resistor 120,compares voltage V_(FIN) with voltage V_(MOUT) appearing at the terminal"b", and outputs an error signal to the integrator 112 in order thatV_(MOUT) becomes equal to V_(FIN).

A current (namely, a current supplied to a DUT) flowing through thecurrent measuring resistor 120 may be detected by measuring a voltageacross ends "a" and "b" of resistor 120. The voltage across ends "a" and"b" is provided as an output voltage from differential amplifier 132 andis then supplied via an ADC (not shown) to the above-describedmeasurement signal processing circuit.

When current setting/voltage measuring operations are executed, asetting current signal (I_(FIN)) is supplied from the above-mentionedmeasurement signal processing circuit via a DAC to error amplifier 111.Error amplifier 111 is fed the voltages appearing across both ends ofresistor 120, and outputs an error signal to integrator 112 in orderthat a current (namely, the current supplied to a DUT) flowing throughresistor 120 becomes equal to the setting current. A voltage applied tothe DUT is detected by measuring a voltage at terminal "b" of resistor120. This voltage is furnished via buffer 131b and an ADC to ameasurement signal processing circuit (not shown).

However, since stabilities of the voltage and current feedback loops areestablished with respect to predictable load conditions in theabove-described circuit, the gain/frequency characteristic of theoverall feedback loop is suppressed to a low value. As a consequence, alengthy time period is required until the output signal V_(OUT)converges, and therefore it is difficult to accomplish a high-speedmeasurement.

Even if a load causing no stability problem is employed, because thefeedback loop is activated based on worst case conditions, measurementspeed is limited.

In other words, when the gain/frequency characteristic of the overallloop is extended to a high frequency range to achieve a high-speedmeasurement, overshoots occur when voltage settings are changed. Aringing phenomenon thus occurs, depending upon load conditions. In aworst case, other problems such as oscillations may occur. Also, sincecapacitors must be connected to various circuit portions to maintainstability of the feedback loop, errors caused by dielectric absorptionand capacitive leakage currents occur.

The present invention solves the above-described problems, and thereforehas as an object to provide a voltage/current measuring unit and avoltage/current measuring method, which can measure various electriccharacteristics (such as a voltage/current characteristic of DUT) athigh-speed and under stable conditions.

SUMMARY OF THE INVENTION

A measuring unit, according to the present invention, is constructed of:

a signal generating source having an error detecting unit;

a current measuring resistor provided between an output terminal of thesignal generating source and a DUT connection terminal;

a voltage measuring circuit for measuring a voltage across the DUTconnection terminal of said current measuring resistor; and

a current measuring circuit for measuring a voltage across the terminalsof said current measuring resistor, the signal measured by said voltagemeasuring circuit or said current measuring circuit being fed back tosaid signal generating source, characterized in that:

an ADC is provided in the voltage measuring circuit and the currentmeasuring circuit. The ADC converts the voltage appearing at the DUT endof said current measuring resistor and the voltage appearing across theboth terminals of the current measuring resistor. The error detectingunit of the signal generating source is digital, and also a DAC isemployed as an output stage of said signal generating source.

Also, a measuring method is employed, wherein a signal from a signalgenerating source having an error detecting unit is inputted to one endof a current measuring resistor whose the other end is connected to aDUT;

a voltage across terminals of the DUT of a said current measuringresistor is measured by a voltage measuring circuit;

a voltage across both terminals of said DUT is measured by a currentmeasuring circuit; and

the signal measured by either said voltage measuring circuit or saidcurrent measuring circuit is fed back to said signal generating source,and said feedback signal is compared with either a current setting valueor a voltage setting value, whereby the voltage appearing across the DUTside of said current measuring resistor is made equal to the currentsetting value, or the voltage appearing across both ends of the currentmeasuring resistor is made equal to the current setting value.

The measuring method is characterized in that both the voltage appearingat the DUT terminal of said current measuring resistor and the voltageacross the terminals of said current measuring resistor are convertedinto digital values in the voltage measuring circuit and the currentmeasuring circuit, and digital error detection occurs between the signalfed back to said signal generating source and either the settingvoltage, or the setting current.

In the signal generating source according to the present invention, theerror detecting unit is constructed as a digital circuit. For instance,an error between the feedback signal and either the setting voltage, orthe setting current is digitally calculated to obtain a digital errorsignal; the digital error signal is D/A-converted into an analog errorsignal; the analog error signal is inputted into the integrator; and anoutput of the integrator is applied to the current measuring resistor.It is of course possible to construct the signal generating sourcewithout providing an integrator at the output of the error detectingunit. In such case, the function (namely, an integral of error values)provided by the integrator may be achieved by a digital signal processor(DSP).

Further, normally, one pair of ADCs are employed for converting theanalog voltage across both ends of the current measuring resistor intodigital values. A digital subtractor calculates a difference between theoutputs derived from both ADCs. After a difference between the analogvoltages across both ends of the current measuring resistor has beenobtained in analog form, the analog difference value may beA/D-converted into a corresponding digital difference value. In suchcase, the above described subtractor may be omitted.

In the present invention, the analog signal derived from the DAC at theoutput of the signal generating source, is supplied via the currentmeasuring resistor to a DUT.

In case of voltage setting/current measuring operations, the analogvoltage appearing across the terminals of the current measuring resistorat the DUT side is A/D-converted into a digital voltage which is thenfed back to the signal generating source. The error detecting unit ofthe signal generating source comprises digital circuits that compare thesetting voltage with the digital voltage signal to change the digitaldata value to be supplied to the DAC in such a manner that the voltageappearing at the terminals of the current measurement resistor at theDUT side becomes equal to the setting voltage.

In case of current setting/voltage measuring operations, voltages acrossboth ends of the current measuring resistor are A/D-converted intodigital voltage signals, respectively. Normally, after a differencebetween both of the digital signals has been digitally calculated, thedigitally calculated difference is fed back to the signal generatingsource as the digital current signal. The error detecting unit of thesignal generating source compares the setting current with the fed backdigital current signal and changes the digital data value supplied tothe DAC in such a manner that the current supplied to the currentmeasuring resistor becomes equal to the setting current.

It should be noted that the voltage measuring circuit A/D-converts thevoltage appearing at the DUT terminal of the current measuring resistor,thereby measuring the current to be supplied to the DUT.

The present invention is preferably operated so that measurements arerepeatedly performed.

For instance, when a measurement is carried out under the same orsimilar conditions to a previous condition (same type of DUT,measurement, environment etc.), the voltage setting value of DAC duringthe previous measurement is stored in an external storage device and thecontrol operation is performed by employing this setting value as astarting point. As a result, the voltage can be set faster than in asemiconductor measurement where 0 V is used as the starting point.

Further, even when no measurement under the same conditions has beenperformed, if the setting value of the DAC enables application of adesired output voltage as a starting point, then a high-speedmeasurement can be achieved that is several times faster than themeasurement where 0 V is used as the starting point.

Furthermore, in case of no repetitive measurement, when the approximatevalue of a DUT is already known, the setting of DAC may be performedwhere the optimum voltage setting value is used as the starting point.

In addition, according to the present invention, an output voltage valuecontrol in accordance with load conditions may be performed. Forinstance, in the conventional unit shown in FIG. 6, it is difficult toobtain a stable output voltage in case of a capacitive load and aninductive load. In contrast, a stable output voltage can be set to adesired condition by varying a digital signal value without changing thecircuit design and the hardware.

Conventionally, when a voltage higher than the rating voltage of a DUTis applied to the DUT, there are some risks that the DUT will bedestroyed. As a consequence, since stability under a worst loadcondition is maintained, there are some cases that the speed ofmeasurement is sacrificed.

In the present invention, since setting of the output voltage in thesignal generating source is performed by a digital circuit which allows,the output voltage to be quickly moved to a desired voltage, theabove-described problems are solved.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a voltage/current measuring unitaccording to an embodiment of the present invention.

FIG. 2 is a block diagram representing a design modification of themeasuring unit shown in FIG. 1.

FIG. 3 is a block diagram indicating another design modification of themeasuring unit shown in FIG. 1.

FIG. 4 is a partial circuit diagram representing a condition when a loadcharacteristic of the measuring unit of FIG. 1 is measured.

FIG. 5 is a graph indicative of an output response in the circuit ofFIG. 4.

FIG. 6 is a circuit diagram for showing the conventional voltage/currentmeasuring unit.

DESCRIPTION OF SYMBOLS

1 - - - signal generating source,

11 - - - DSP,

12 - - - DAC,

13 - - - buffer,

2 - - - current measuring resistor,

3 - - - current measuring circuit,

31a, 3lb - - - buffer,

32a, 32b - - - ADC,

33 - - - digital subtracter,

4 - - - voltage measuring circuit.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a circuit diagram of a measuring unit according to oneembodiment of the present invention. A signal generating source 1includes a digital signal processor (DSP) 11, DAC 12 and a buffer 13.DSP 11 includes an error detecting unit, into which a voltage settingvalue V_(FIN) is supplied in the form of a digital signal. Also digitalsignals are inputted from ADC 32b and a digital subtracter 33 (to bediscussed below).

A buffer 13 is connected to one terminal "a" of a current measuringresistor 2, and another terminal "b" of current measuring resistor 2 isto a DUT.

Terminals "a" and "b" of resistor 2 are respectively connected viabuffers 31a and 31b to ADC 32a and 32b. Output terminals of both ADCs32a and 32b are connected to input terminals of a digital subtracter 33.

As previously explained, output terminals of ADC 32b and subtracter 33are connected to DSP 11. It should be noted that outputs of ADC 32b andthe subtracter 33 are transferred as a voltage measurement valueV_(MOUT) and a current measurement value I_(MOUT), respectively, to ameasurement signal processing circuit (i.e., a microprocessor, memory,etc., not shown).

Buffers 31a, 31b, ADC's 32a, 32b and subtracter 33 constitute a currentmeasuring circuit 3. Also, buffer 31b and ADC 32b constitute a voltagemeasuring circuit 4.

In the case of voltage setting/current measuring operations, an analogvoltage appearing at terminal "b" of current measuring resistor 2 isinputted to ADC 32b via buffer 31b. In ADC 32b, the analog voltage isconverted into a digital signal V_(MOUT). The digital signal is suppliedto a measurement signal processing circuit (not shown) and also to DSP11. In DSP 11, digital value V_(MOUT) is compared with voltage settingvalue V_(FIN). Both a difference value between the value V_(MOUT) andvoltage setting value V_(FIN) and past historical data of the differencevoltage are used to calculate an optimum setting value for DAC 12. Inother words, a digital value is calculated that causes output voltageV_(OUT) to be driven to equal voltage setting value V_(FIN).

Current measuring circuit 3 measures a current supplied to the DUT,based on a voltage measured value I_(MOUT) between both terminals of thecurrent measuring resistor 2, and also the resistance value thereof.

In case of current setting/voltage measuring operations, an analogvoltage value derived from both terminals "a" and "b" of currentmeasuring resistor 2, is A/D-converted by ADCs 32a and 32b. The digitaloutputs from both ADCs 32a and 32b are inputted into subtracting circuit33 to calculate a digital difference therebetween. Thereafter, thedigital difference value is fed back to DSP 11 as a current signalI_(mout).

DSP 11 compares the feedback current signal I_(MOUT) with settingcurrent I_(FIN), and derives a digital value which is supplied to DAC 12so that current supplied to the current measuring resistor 2 (namely,current supplied to the DUT) becomes equal to the setting currentI_(FIN). The voltage measuring circuit 4 measures the voltage applied tothe DUT by deriving the voltage appearing at the DUT terminal "b" ofcurrent measuring resistor 2 and then by A/D-converting this voltage.

As a result, various characteristics of a DUT, such as a voltage/currentcharacteristic, may be stably measured at high speed.

FIG. 2 shows an example of a modification of the unit indicated inFIG. 1. In the unit of FIG. 2, a voltage across both terminals "a" and"b" of current measuring resister 2 is obtained as an analog value by adifferential amplifier 34. Then, the analog voltage is A/D-converted byADC 35 into a digital voltage which is then fed back to DSP 11 (in thiscase, buffer circuit 31a and subtracting circuit 33 are not needed).

In FIG. 2, current measuring circuit 3 includes differential amplifier34 and ADC 35. The voltage measuring circuit 4 includes a buffer 41 andADC 42 (corresponding to buffer 31a and ADC 32 of FIG. 1). Voltagemeasuring circuit 4 is used to measure a voltage appearing at terminal"b" of current measuring resistor 2. Different from the voltagemeasuring circuit of FIG. 1, voltage measuring circuit 4 is not soconstructed to use a portion of current measuring circuit 3.

Because ADC 35 A/D-converts a differential voltage across terminals "a"and "b" of current measuring resistor 2, resolution is increased.

Since the above-described unit derives an analog difference value by wayof differential amplifier 34, an analog error may occur. However, solong as this error is negligible, the unit shown of FIG. 2 is preferablyutilized.

FIG. 3 indicates another example of a design modification of the unitshown in FIG. 1.

In the unit of FIG. 3, signal generating source 1 includes DSF 11', DAC12', an integrator 14 provided at the output of DAC 12, and a buffer 13.DSP 11' detects an error between a setting signal and a feedback signal.An error component is converted by DAC 12' into an analog signal, andthereafter this analog signal is integrated by integrator 14.

In the arrangement of FIG. 3, the integrating process of the errorcomponent effected in DSP 11 in the unit of FIG. 1 is performed by ananalog integrator 14. Since an integrator includes no minimumcalculating unit (as in the DSP processing apparatus), resolution of theoutput voltage is not limited by resolution of DSP 11 and DAC 12, as incase of FIG. 1. As a result, the resolution of DAC 12' is reduced to theminimum 1 bit, thereby simplifying the hardware arrangement.

As previously stated, controls adapted to load conditions can beperformed by the DSP.

FIG. 4 is a partial circuit diagram showing a load connected to themeasured signal output terminal of the unit shown in FIG. 1, in order tomeasure the load characteristic of this unit.

In FIG. 4, a stepped waveform is inputted from DAC 12 via buffer 13 andcurrent measuring resistor 2 to a DUT (not shown). The distortedwaveform is shown at output terminal "b". It should be noted that straycapacitance of a fixture (measuring jig) is indicated by referencenumeral 100.

Generally speaking, there are many possibilities in a measurement of asemiconductor that the DUT may be represented as a capacitive load.Also, even if a DUT per se does not exhibit a capacitive characteristic,there is capacitance from the measuring jig (see capacitance 100 of FIG.4). This capacitance will constitute a time constant RC (symbol "R"denotes a resistance value of resistor 2, and symbol "C" represents thecapacitance value 100). This time constant causes a lag in a rising edgeof a waveform appearing at the measurement terminal (see in FIG. 5).

In FIG. 4, DSP circuit 11 provides no correction based on a differencebetween the voltage at terminal "b" and the setting voltage value, whichis different from normal operation, however, a simple stepped voltagewaveform is outputted.

The voltage waveform at terminal "b" is shown in FIG. 5. This waveformis observed at the ADC (see ADC 32b of FIG. 3) that is provided at theoutput side of buffer 31b. The time constant "RC" is given as follows,assuming that differences between the fixed value and the voltage valueat time instants t₁ and t₂ are X₁ and X₂, respectively;

RC=(t₂ -t_(l))/(log x₂ -log x_(l)), where the symbol "log" indicates anatural logarithm.

It should be noted that since the resistance value "R" is known, thecapacitance value "C" may be determined from this value "R".

In accordance with this measurement, either the integrating timeconstant "T" for the DSP digital feedback process, or the stablecondition of the gain 1/T may be obtained under such conditions thatT>RC, or 1/T<1/RC.

If the actual digital feedback gain is set with a proper allowancewithin the range for satisfying this condition, then a high-speedsetting operation can be stably achieved.

Furthermore, in measuring units according to the present invention(especially, as shown in FIG. 1 and FIG. 2), no analog integrator iscontained in the signal generating circuit), an arbitrary numeral seriesis given to the DAC for driving the output buffer, so that this DAC maybe used as an arbitrary waveform generator (AWG).

It should be noted that in a conventional measuring unit such as shownin FIG. 6, since there is provided an integrator between the V_(FIN) andthe output, the DAC cannot directly drive the measuring terminal of aDUT (as an arbitrary waveform generator) with a sufficient speed.

In semiconductor measurements, it is important to measure an appliedbiasing voltage and a capacitance when a DC bias is applied to thesemiconductor. Measurements of the DC bias voltage and the capacitanceof DUT are referred to as a "C-V measurement".

A relationship between C and V (i.e., C-V characteristic) may beobtained, without separately providing hardware for measuringcapacitance, by utilizing the present invention. That is to say, themeasuring unit is used as an arbitrary waveform generator withoutemploying digital feedback. A sine wave superimposed with a DC voltageis produced by this arbitrary wave generator, and the measurements ofvoltage and current for a DUT are performed by the ADC in synchronismwith the generated sine wave.

It should be understood that the foregoing description is onlyillustrative of the invention. Various alternatives and modificationscan be devised by those skilled in the art without departing from theinvention. Accordingly, the present invention is intended to embrace allsuch alternatives, modifications and variances which fall within thescope of the appended claims.

We claim:
 1. A voltage/current measuring unit comprising:a signalgenerating source including a digital signal processor having a digitalerror detecting unit, and a digital-to-analog converter (DAC), connectedin series, said digital error detecting unit providing a digital errorvoltage to said DAC that is a difference between settable input digitalvoltage and feedback digital signal values; a current measuring resistorhaving first and second terminals respectively connected to an outputterminal of the signal generating source and a connection terminal; avoltage measuring circuit connected to said digital error detecting unitfor measuring a voltage at said second terminal of said currentmeasuring resistor and for providing a voltage feedback signal to saidsignal generating source; and a current measuring circuit connected tosaid digital error detecting unit for measuring a voltage across saidfirst and second terminals of said current measuring resistor, and forproviding a current feedback signal to said signal generating source;said voltage measuring circuit and current measuring circuit bothincluding analog-to-digital converter means for respectively concerninga voltage appearing at said second terminal of said current measuringresistor and a voltage appearing across said first and second terminalsof said current measuring resistor int digital feedback voltage valuesthat are fed to said signal generating source as said feedback digitalsignal values.
 2. The voltage/current measuring circuit as recited inclaim 1 wherein said digital error detecting unit outputs an initialdigital error voltage value dependent upon a pre-established parameterderived from a past output voltage.
 3. The voltage/current measuringcircuit as recited if claim 1 wherein said digital error detecting unitoutputs an initial digital error current value dependent upon apre-established parameter derived from a past output current value. 4.The voltage/current measuring circuit as recited in claim 1 wherein saidvoltage measuring circuit comprises a buffer amplifier and an analog todigital converter (ADC) and said current measuring circuit comprisessaid voltage measuring circuit and a series connected buffer amplifier,ADC and a digital subtractor, said digital subtractor also fed by anoutput from said ADC in said voltage measuring circuit and providing adigital difference output to said digital error detecting unit.
 5. Thevoltage/circuit measuring circuit as recited in claim 4 wherein saidsignal generating source further includes intergrating means coupledbetween anoutput from said DAC and said first terminal.
 6. Thevoltage/current measuring circuit as recited in claim 1 wherein saidvoltage measuring circuit comprises a buffer amplifier and a seriesconnected analog-to-digital converter (ADC), and said current measuringcircuit comprises a buffer amplifier and a series connected bufferamplifier and ADC, outputs from said ADC's connected to said digitalerror detecting unit.
 7. A voltage/current measuring method wherein asignal from a signal generating source having an error detecting unit isinputted to a first terminal of a current measuring resistor, saidcurrent measuring resistor having a second terminal connected to anobject to be measured, the method comprising the steps of;measuring avoltage at said second terminal of said current measuring resistor by avoltage measuring circuit which provides a digital voltage measurementvalue; measuring a voltage across said first and second terminals ofsaid current measuring resistor by a current measuring circuit whichprovides a digital current measurement value; feeding said digitalvoltage measurement value from said voltage measuring circuit, or saiddigital current measurement value from said current measuring circuit tosaid signal generating source; and comparing said digital currentmeasurement value with either a digital current setting value, or adigital voltage setting value, as the case may be, to provide an outputwhich causes a voltage appearing at said second terminal of said currentmeasuring resistor to approach the current setting value, or the voltageappearing across the first and second terminals of the current measuringresistor to approach the digital voltage setting value.